The ying/yang relationship of differentiation versus proliferation is a fundamental tenet in the development of multicellular organisms, as well as the basis for cancer. Transcription factors (TFs), such as nuclear receptors (NRs), regulate the genes responsible for carrying out these cellular processes. HNF4? is a highly conserved member of the NR superfamily of ligand dependent TFs. During the past 20+ years, HNF4? has been well characterized for its role in early development, liver differentiation and an inherited form of diabetes (MODY1). More recently, a role for HNF4? has emerged in the intestine/colon and several links to cancer have been reported. Intriguingly, the role for HNF4? in cancer seems to depend on the particular isoform of HNF4?: there are nine isoforms driven by two alternative promoters, P1 and P2. Here, we propose to follow up on the results from the previous round of funding in which we identified the mechanism responsible for differential regulation of the P1- and P2-driven HNF4? isoforms in human colon cancer. That mechanism involves the oncogene Src tyrosine kinase and is impacted by single nucleotide polymorphisms (SNPs) in the HNF4? gene. We will pursue the following three Specific Aims: 1) Determine the role of the P1- versus P2- HNF4? isoforms in the colon by using mouse models, human culture models and human tissue analysis; 2) Determine the role of P2- HNF4? in liver cancer and development by using mouse models and cutting edge -omic approaches to decipher the molecular basis for the differences in the isoforms; and 3) Elucidate the effects of the Src tyrosine kinase pathway on HNF4? using mouse and human models. The proposed experiments will provide invaluable insights not just into the molecular mechanisms of cancer but also into the dichotomy between the differentiation and proliferation and molecular mechanisms of nuclear receptor action. They will generate results that could be of use in the clinic to help properly diagnose and treat liver and colon cancer.